Stoker for the combustion of coal



Jan. 14, 1964 E. R. MITCHELL ETAL 3,117,537

STOKER FOR THE COMBUSTION 0F COAL 2 Sheets-Sheet 1 Filed Deo. 13, 1960 ,n un. em wd r we om/d. Q dm mw l un im Jan- 14, 1964 E. R. MITCHELL ETAI. 3,117,537

STOKER FOR THE COMBUSTION OF COAL Filed De'c. 15, 1960 2 Sheets-Sheet 2 W l j so is 5" 450 57 United States Patent O 3,117,537 STKER FR THE CMBUSTIGN F CQAL Earland Roe Mitchell and Frank Donald Friedrich, Ottawa, Ontario, Canada, assignors to Her Majesty the Queen in right of Canada as represented by the Minister of Mines and Technical' Surveys Filed Dec. 13, 1960, Ser. No. 75,570 Claims priority, application Canada .'lan. 29, 1960 4 Claims. (Cl. 110-52) This invention relates to a method for the substantially smokeless combustion of coal and an apparatus for carrying out this method.

Some coals of which those mined in Nova Scotia and New Brunswick are typical, are diiicult to burn on conventional grates without producing excessive smoke. These coals are ditlicult to burn because of their plasticity on ignition, strong coking characteristics, high volatile content and the low fusion temperature of their ash. Conventional stokers will not eiciently handle these coals. If with a conventional Stoker, the layer of coal fed on to the grate exceeds about two inches, excessive smoke will be produced and if, on the other hand, the coal is fed onto the grates in layers less than about two inches, the thin tire bed which results provides insuiiicient thickness of ash to insulate the grate in the combustion zone, from the heat of the fire or furnace. The end result is burning of the grate surface unless coal burning rates are reduced too low to be economic or competitive with other fuels.

In accordance with the method and apparatus of this invention, a Stoker is provided for the substantially smokeless combustion of coal comprising a first grate stage, means for feeding coal to said first grate stage, a second grate stage, means for causing the coal to travel from the rst grate stage to the second grate stage, means for removing ash and residue from the second grate stage, means for directing air from combustion up through the coal at the second grate stage, suction means for drawing a proportion of the products of combustion of the second grate stage down through the coal at the first grate stage to pre-oxidize and partially gasify and devolatilize the coal at the first grate stage to a suiiicient extent to prevent the coal becoming plastic when it reaches the second grate stage, and to provide a stream of combustible gas, means for directing said stream of combustible gas over the coal at the second grate stage, to provide over fire turbulence and a substantially complete burning of the combustible gases evolved by said coal. Preferably a vibrating grate is used, divided into first and second stages.

A construction in accordance with this invention possesses marked advantages in comparison with conventional stokers. The first stage provides controlled preoxidizing of the raw coal by partial gasification or devolatilization of the coal, without producing smoke in the furnace and reduces the tendency of some coals to become plastic during the combustion in the second stage. The use of suction to draw part of the products of combustion of the second stage down through the first stage, causes flame from the tire to sweep towards the raw incoming coal, with the advantage that the conventional rear refractory arch in the furnace can be eliminated or shortened. Furnace construction and maintenance costs are therefore decreased. The combustible gas resulting from the downiow through the first stage of the grate is directed over the coal in the second stage providing over fire turbulence which ignite and completely burns without excessive dilution with air unburned furnace combustibles, which are the source of smoke. More complete combustion is provided than in conventional stoxers and heat losses are reduced because the turbulent area 3,117,537 Patented Jan. I141, 1964 ice over the second stage will result both in the combustion of the combustible gases evolved from the first stage and the combustion of combustible gases evolved during the second stage. The foregoing may be achieved without the introduction of additional air.

Referring now to the drawings which illustrate the preferred embodiment of this invention.

FIGURE 1 is an elevation View in section of a construction in accordance with the invention,

FIGURE 2 is an elevation detailed view illustrating a modification of the construction shown in FIGURE l,

FIGURE 3 is a detailed elevation view showing the operation of the ram feeder shown in FIGURE l,

FIGURE 4 is a perspective elevation view illustrating a grate vibrating mechanism suitable for use in conjunction with the apparatus shown in FIGURE 1,

FIGURE 5 is a detailed view of the eccentric weight shown in FIGURE 4.

Referring now to FIGURE 1 of the drawings, the apparatus illustrated comprises a boiler furnace 10 tired by the combustion of coal on a vibrating grate 11. Coal 12 is fed to the upper end 13 of vibrating grate 11 from coal hopper 14, by means of ram feeder 15. The coal travels down along the length of vibrating grate 11 to ash pit 16. Cooling Water to prevent the grate from rising to an excessively high temperature is circulated through the grate through liexible pipe 17 and pipe 18. Air compartments 19 are supplied with combustion air from air blower 20 and direct combustion air up through the second stage of the vibrating grate. Air compartments 19 are supplied with dust catchers 21 to collect any ash which may drop into the air compartments. The portion of the grate which is above air compartments 19 constitutes the second stage of the grate. A dust separator 22 provides a compartment above which is the first stage of the grate. Gas blower 23 is connected with dust separator 22 to exert suction on the first stage portion of the grate causing part of the hot products of combustion of the second stage to be drawn downwardly and through the coal in the lirst stage. The temperature of the products of combustion above the second stage of the grate may for example be about 800 C., to l,000 C., and the temperature of the coal in the iirst stage of the grate may average for example about 200 C. to 400 C. It will be appreciated however, that there will be considerable variation depending on the best operating conditions for the particular apparatus and coal. These products of combustion or furnace gases together with additional volatiles and combustible gases which are dissociated from the raw coal by the hot furnace gases in the first stage, are driven by gas blower 23 through duct. 24 to over-fire gas jets 25 where they are directed downwardly across the second stage to produce over-fire turbulence and substantially complete burning of the combustible gases evolved both during the first and second stages.

In the modification illustrated in FIGURE 2 gas blower 23 has been replaced with a steam injector system. An inspirated gas line 30 extends into the compartment provided by dust separator 22. A high pressure steam line 31 is connected to a steam injector nozzle 32 which is located within over-tire gas jet 25a to create suction within inspirated gas line 30 and to cause a mixture of gas and steam to be directed over the first stage. Preferably a series of steam injector nozzles 32 is provided extending across the Width of the Stoker and corresponding with a series of over-lire gas jets fed by gas header 33.

FIGURE 3 is a detailed view of the ram feeder in FIGURE 1, which comprises a feeder plate 40 arranged to reciprocate at the bottom of hopper 14 and above the upper end of grate 11. A hinged seal plate 41 is provided at the bottom of hopper 14 to prevent ycoal from escaping while permitting the reciprocating movement to occur. A lever arm 42 is connected at one end by pivot pin 43 to feeder plate 4t), is pivoted by pivot pin 44 and is reciprocated by a steam or hydraulic piston or other power drive 45, which reciprocates rod 4d connected to the other end of lever 42 by pivot pin 47. Preferably, pivot pin 47 is engageable with one of a selection of apertures 48 in the lower end of lever arm 42 to provide an adjustment of the feeder plate stroke.

Any suitable mechanism may be used for vibrating the grate. FGURES 4 and 5 illustrate one suitable mechanism in which bearing blocks 50 carrying journal members 51 secured to Stoker framework 52 upon which vibrating grate 11 is mounted. A shaft 53 having circular portions 54 and 55 and a square portion 56 is journalled in journal portions 51 and is driven by V-belt 5'7 and pulleys 58 and 59. An electric motor 60 drives pulley 5S through a transmission 61. Transmission 61 should include means for adjusting the rotational speed and may for example be a Reeves drive. An adjustable eccentric weight 62 is mounted on the square portion 56 of shaft 53 and is shown in greater detail in FIGURE 5, comprises an eccentric weight 63 and a slot 64. Allen screws 65 and 66 can be used for adjusting the eccentricity of the weight.

Where a vibrating grate is used, a suitable frequency for the vibration of the grate will be about 1200 cycles per minute and a typical amplitude will be about 1A; of an inch.

We claim:

l. A Stoker for the substantially smokeless combustion of coal comprising a grate, means for feeding coal to one end ot" said grate, means for causing the coal to travel along the length of said grate, means for removing ash and residue at the other end of said grate, means for dividing the length of said grate into two sections, the first section being adjacent to said feeding means and the second section being adjacent to said other end of the grate, means for continuously directing air for cornbustion up through the coal at the second section of said grate, suction means for continuously drawing a proportion of the products of combustion of the second section down through all the coal at the first section to pre-oxidize and partially gasify and devolatilize the coal at the first section, to a suicient extent to prevent the coal becoming plastic when it reaches the second section and to provide a stream or combustible gas, means for directing substantially the entire stream of combustible gas over the coal in the second section to produce overre turbulence and a substantially complete burning of the combustible gases evolved by said coal.

2. An apparatus as in claim l in which said grate is a vibrating grate having means for vibrating said grate with a frequency of the order of 1200 cycles per minute.

3. An apparatus as in claim 1, in which the means for directing said stream of combustible gas over the coal in the second section, comprises a gas blower constructed and arranged to direct gases withdrawn from the rst section to over-tire gas jets, said over-lire gas jets being directed downwardly and transverse to the rst section.

4. An apparatus as in claim 1, in which said means for directing said stream of combustible gas over the coal comprises over-lire gas jets, steam injector nozzles within said over-lire gas jets and an inspirated gas line arranged to withdraw gases from beneath said rst section.

References Cited in the le of this patent UNITED STATES PATENTS 2,763,221 Miller Sept. 18, 1956 2,913,912 Raderrnacher Nov. 24, 1959 FORElGN PATENTS 157,708 Great Britain Feb. 9, 1922 448,495 Great Britain June 9, 1936 635,692 Germany Sept. 19, 1936 928,358 Germany May 3l, 1955 

1. A STOKER FOR THE SUBSTANTIALLY SMOKELESS COMBUSTION OF COAL COMPRISING A GRATE, MEANS FOR FEEDING COAL TO ONE END OF SAID GRATE, MEANS FOR CAUSING THE COAL TO TRAVEL ALONG THE LENGTH OF SAID GRATE, MEANS FOR REMOVING ASH AND RESIDUE AT THE OTHER END OF SAID GRATE, MEANS FOR DIVIDING THE LENGTH OF SAID GRATE INTO TWO SECTIONS, THE FIRST SECTION BEING ADJACENT TO SAID FEEDING MEANS AND THE SECOND SECTION BEING ADJACENT TO SAID OTHER END OF THE GRATE, MEANS FOR CONTINUOUSLY DIRECTING AIR FOR COMBUSTION UP THROUGH THE COAL AT THE SECOND SECTION OF SAID GRATE, SUCTION MEANS FOR CONTINUOUSLY DRAWING A PROPORTION OF THE PRODUCTS OF COMBUSTION OF THE SECOND SECTION DOWN THROUGH ALL THE COAL AT THE FIRST SECTION TO PRE-OXIDIZE PARTIALLY GASIFY AND DEVOLATILIZE THE COAL AT THE FIRST SECTION, TO A SUFFICIENT EXTENT TO PREVENT THE COAL BECOMING PLASTIC WHEN IT REACHES THE SECOND SECTION 